This invention relates to a recording device for recording, in a track form, information such as music signals, etc. on a recording medium such as an optical disk, etc., and more particularly to a device for processing signals indicative of recorded information.
Recording media such as an optical disk, etc. have a so called track structure in which recording is conducted by signal pits arranged in a spiral manner from the inner circumferential side to the outer circumferential side.
In an information recording/reproducing device using a recording medium with such a track structure, a transmission type optical disk device is known. Such a transmission type optical disk device functions as follows. At the time of recording, a signal source modulates a signal to be recorded (hereinafter referred to as "an original signal") in accordance with an amplitude modulation system to output a modulated original signal in the form of a time-series signal. The outputted original signal is converted into a laser beam corresponding to the intensity of the original signal by a cutting unit. The modulated light signal thus obtained is converged and the optical disk is then exposed to the modulated light signal. In this way, the original signal is recorded on a medium in a spiral manner in the form of trains of bright and dark photosensive areas corresponding to the intensity of the original signal.
On the other hand, at the time of reproduction of the medium, the modulated signal is read by the pickup unit. A laser beam emitted from a semiconductor laser is converged on the recording surface of the optical disk by an objective lens. The diameter of a light spot formed on the recording surface of the optical disk by convergency of the laser beam is determined at a value that it is irradiated only onto a track to be traced (target track) but is not spread over other adjacent tracks. This prevents occurrence of leakage or mixing of a read signal from adjacent tracks into the target track. The laser beam thus irradiated is transmitted through the optical disk and is then incident to a photodetector provided on the opposite side of the semiconductor laser. The read signal which has been subjected to photoelectric conversion is reproduced.
The problem with the above-mentioned conventional recording/reproducing device is that recording density cannot be increased to a value more than the diameter of the light spot.
Namely, it is conceivable to narrow the spacing or interval between adjacent tracks in order to increase the recording density. The diameter of the light spot must be reduced in correspondence with the reduction of the interval between adjacent tracks. However, the diameter of the light spot is restricted by the performance (converging ability) of the objective lens, the wavelength of the laser beam, and the like. When an attempt is positively made to reduce the diameter of the light spot, it is required to use an objective lens having a high converging ability, and it is also required to conduct the tracking or the focusing control with a high accuracy. Such requirements result in disadvantages such that the device becomes complicated and/or its cost is increased.